Method of removing hydrogen sulfide from gaseous mixtures

ABSTRACT

The process for selectively removing H2S and like sulfides from fluids containing them by contact with a cyanopyridine (e.g., a mixture of ortho and meta cyanopyridines) and an alkali hydrosulfide, preferably in a substantially hydroxyl-free solvent such as N-methyl pyrrolidone. Preferably, in the process an admixture of H2S and CO2 in natural gas is contacted with the cyanopyridine containing contacting solution to react the H2S with said cyanopyridine, the CO2 and/or like hydrocarbons are rejected from the contacting solution by mild heating and/or pressure reduction and thereafter H2S is regenerated by heating the remaining solution.

United States Patent [151 3,656,?

Suzuki et al. [451 Apr. 1, 1972 [54] METHOD OF REMOVING HYDROGEN3,120,993 2/1964 Thormann et a1. ..23/2 SULFIDE FROM GASEOUS MIXTURES3,242,646 3/1966 Miller et a1 ..55/73 x [72] Inventors: Shigeto Suzuki,San Francisco; Giok H.

Tjoa, Placentia; Karl H. Kilgren, La Prlmary Exammer Earlc'ThomasAttorney-A. L. Snow, F. E. Johnston, C. .l. Tonkin and G. F. l-labra,all of Calif. Magdeburger [73] Assignee: Chevron Research Company, SanFrancisco, Calif. [57] ABSTRACT Filedl g- 21,1969 The process forselectively removing H 5 and like sulfides [21] AppL NM 851,871 g fromfluids containing them by contact with a cyanopyridine (e.g., a mixtureof ortho and meta cyanopyridines) and an alkali hydrosulfide, preferablyin a substantially hydroxyl-free UOSI Cl. .-23/2 R, E olvent uch aspyrrolidone Preferably in the [51] Int. Cl. ..B01d 53/16, BOld 53/34process an admixture f 5 and CO2 in natural gas is Com 58 FM fS h23/2233334181' 1 0 care r tacted with the cyanopyndme containingcontacting solunon 55/73; 260/5515 294's E to react the H 8 with saidcyanopyridine, the CO and/or like hydrocarbons are rejected from thecontacting solution by [56] References Cited mild heating and/orpressure reduction and thereafter H 8 is UNI-[ED STATES PATENTSregenerated by heating the remaining solution. 1,580,451 4/1926 Sperr,Jr. ..23/1 81 8 Claims, 1 Drawing Figure Co -FREE GAS co ABSORBER FLASHDRUM v 19 2 REGENERATOR H25 REACTOR a 29 I3 3 24 l8 1/ I II x 27 SOURGAS u ,7 a

I m FLASH FLASH REFLUX DRUM DRUM DRUM METHOD OF REMOVING HYDROGENSULFIDE FROM GASEOUS MIXTURES BACKGROUND OF THE INVENTION This inventionrelates to the selective removal of sulfides such as hydrogen sulfidefrom fluids by contacting such fluids with cyanopyridine andhydrosulfide in an anhydrous solvent. The invention has particularapplication to the selective removal of hydrogen sulfide from admixturesof normally gaseous hydrocarbons, carbon dioxide and hydrogen sulfide.

Natural gas often contains appreciable amounts of H 8 along with CO andnormally gaseous hydrocarbons and it is desirable to remove H 8 fromnatural gas and similar fluids. Also, it is important to remove sulfidessuch as mercaptan, as well as hydrogen sulfide, to produce fluideffluents of low residual sulfur content. In many instances, H must beremoved in order to meet pipeline specifications (for example, to amaximum of 0.25 grains of H 8 per 100 SCF of gas), but the simultaneousremoval of CO is often unnecessary or undesirable.

While a multiplicity of known processes is available for the removal ofacidic constituents from gas streams, all can be classified under one ora combination of four major groupings as follows: (I) processesinvolving an acid-base neutralization wherein the gaseous acidiccomponent is converted to a salt (i.e., neutralization process); (2)processes involving physical solution wherein the gaseous acidiccomponent is dissolved in a liquid solvent in accordance with theprinciple of Henrys law and no chemical reaction takes place (i.e.,physical solution process); (3) processes involving physical permeationand adsorption of the gaseous acidic component within the pores of asuitable, solid absorbent material (i.e., adsorption process); and, (4)processes involving an oxidation reaction wherein H 8 and certainsulfides are oxidized to elemental sulfur or a higher state of oxidationwhile CO being in its highest state of oxidation is nonreactive (i.e.,oxidation process). Each of the foregoing groups of processes hascharacteristic properties which may be used to delineate or predict theadvantages and disadvantages of any individual process included withinthe classification.

Thus, the neutralization processes are essentially nonselective for H Sor CO since both components are acidic and readily neutralized by achemical base. The alkanolarnine process, which is the most commonlyemployed regenerative neutralization process, utilizes water for asolvent and operates at acid gas loadings less than stoichiometricallyattainable to reduce corrosion of processing equipment. Further, thechemicalreaction constant for alkanolamines and H 8 is such that in mostinstances it is economically impractical to produce a treated gas whichmeets pipeline specifications regarding H 8 at pressures significantlyless than 100 psi because of the increased reagent circulation andregeneration requirement.

Physical solution processes may show some selectivity for H 8 relativeto CO but the relative preference for H S is generally limited toseveralfold rather than one or more orders of magnitude. Furthermore,the actual amount of CO removed from the treated gas stream can besubstantially larger than the quantity of H 8 removed, depending ontheinitial concentrations of each component in the original gas stream.In general, it is economically impractical to produce a treated gaswhich meets pipeline H 8 specifications with a physical solution processbecause of the high degree of solvent regeneration and high rate ofsolvent circulation required. Finally, CO and hydrocarbon componentsdissolved in the solvent solution cannot be rejected by flashing therich solvent to a lower pressure or heating the rich solvent to a'higher temperature, because substantial quantities of H S would beliberated concurrently in accordance with .established equilibriumdistribution ratios.

Physical permeation/adsorption processes are quite similar to physicalsolution processes with respect to the selective .removal of H 5 from agas stream containing both H 8 and C0,. The difierence in size of an H 8molecule (3.1 A.) and a C0 molecule (3.8 A.) when compared to thevariation in pore sizes of treated synthetic zeolites or molecularsieves is insufficient to ensure more than a modest increase in theratio of H 8 to C0, removed. Similarly,'the actual quantity of COco-adsorbed can be substantially large, depending on its initialconcentration in the gas stream, thereby reducing the capacity of themolecular sieve for H 8. Furthermore, permeation/adsorption processesmust be operated in a batchwise manner which is undesirable from thestandpoint of equipment duplication and recycle or disposal of the sourregeneration gas stream. Finally, this method of processing iscapacity-limited and generally restricted to either low H 8concentrations, or small sour gas volumes because of physicallimitations on vessel diameters and investment costs for equipment.

Oxidation processes are truly selective or specific for removing H 5 andcertain alkyl sulfides from admixtures of CO and natural gas. Gaseous H8 is generally converted to elemental sulfur by any of a variety ofoxidizing agents suspended on a solid support or dissolved in a suitablesolvent. The iron-sponge or dry-box process, which is the most commonlyused solid support process today, is a batchwise process and subject tovery similar equipment duplication and capacity limitations cited forthe physical permeation/adsorption process. In addition, sulfur isgenerally not recovered in this process but is thrown away with thespent bed material which must be replaced periodically at substantialinconvenience to replenish the active oxidant. Liquid phase oxidationprocesses operate in a continuous manner and sulfur is generallyrecovered as a salable product. However, these processes in general areplagued with plugging problems in flow lines and elements, since sulfuris precipitated as a solid in the liquid phase. Additional techniquesand equipment such as flotation chambers and filters which are notcommonly used or understood by the natural gas industry are alsorequired. Finally, some processes (Giammarco-Vetrocoke, Thylox) employhighly toxic oxidizing agents (arsenic oxides and thioarsenates)potentially detrimental to the safety of operating personnel and publicat large.

SUMMARY OF THE INVENTION jected or desorbed by flashing the soursolution at a lowerpressure and/or higher temperature.

By means of the process of the present invention the H 5 and othersulfides are reacted with the contacting solution, thereby producing afluid effluent of substantially reduced sulfide content. A majoradvantage of the present process is that no matter what the-initialconcentration of H 8 is, the process can remove essentially all the H 8present, i.e., down to the 'last traces of H 8. Thus, pipelinespecification gas (less than 0.25 grain H S/ SCF of gas) or essentiallycompletely H S- free gas (i.e., less than 0.01 grain H S/ 100 SCF ofgas) can be produced by the present process.

Additional advantages of the process are that it can be conducted atrelatively low pressures such as atmosphericpressure or evensubatmospheric pressure, and the contacting can be carried out atrelatively high temperatures. Also, the nonaqueous contacting solutionsare relatively noncorrosive. The enriched contacting solution isreadily. regenerated by heating with or without the aid of an inertstripping gas.

It is believed that the process involves the reaction of thesulfideswith the cyanopyridine to form thioamide in the case of H 8removal or thioamide derivatives in the case of removal of mercaptans.This differs from absorption of H in benzonitrile or in N-methylZ-pyrrolidone (US. Pat. No. 3,120,993). H 8 absorbed in the foregoingcomponents is released at least in part upon pressure reduction and/ormild heating, whereas H S when reacted within stoichiometric limits withthe cyanopyridine in the liquid contacting solution of the presentinvention is not evolved upon pressure reduction or mild heating, i.e.,below the point of decomposition of the reaction product of thecyano-pyridine and the sulfide (e.g., below 250 F. when using a mixtureof ortho and meta cyanopyridine in a solvent of N-methyl 2-pyrrolidone).In accordance with the present invention, the contacting solution canremove sulfides more completely than the so called physical absorbentsor solvents which are limited by an equilibrium between the partialpressure of the sulfide in the gas phase and its concentration in theliquid phase as described by I-lenrys law. When the cyanopyridine H Sratio is greater than stoichiometric, essentially all the CO andhydrocarbons can be flashed off (by pressure reduction and/or mildheating) without release of any appreciable amount of H 8.

The cyano-pyridines employed in the contacting solution of the presentprocess have a relatively high chemical reactivity toward H 8 andsimilar organic sulfides. The cyanopyridines can have ring substituentssuch as additional cyano substituents. Other suitable substituentsinclude electron-attracting substituents such as COOH, Cl, Br, I, F, andthe like. Hence, suitable cyanopyridines include para-cyanopyridine,ortho-cyanopyridine, meta cyanopyridine, 2-chloro, 3- cyanopyridine,3-bromo, 4-cyanopyridine, 2,6- dicyanopyridine, and 5-methyl,B-cyanopyridine. Derivatives of the cyanoyridines can be employedprovided that the additional groups such as methyl ring substituents donot interfere (e.g., by steric hindrance) with the desired reaction. Thedicyanopyridines other than vicinal dicyano pyridine (e.g., 2,3 or3,4-dicyanopyridines) are preferred for their regenerability. Orthocyanopyridine is preferred and a mixture of ortho and meta cyanopyridineis especially preferred in view of its high reactivity, high capacity,regenerability of its reaction products, solubility and thermalstability.

Sufficient cyanopyridine is brought into contact with thesulfide-containing feed to react with at least an appreciable proportionof the sulfides present. In continuous contacting systems, the feedrate, the contact time, the rate of circulation of contacting solutionand the concentration of cyanopyridine in the contacting solution allbear on the ratio of cyanopyridine to sulfide. Ideally for completesulfide reaction, a stoichiometric ratio of cyanopyridine to H S (orequivalent S compound to be reacted) would be used, but practically aslight excess is normally used. The stoichiometric ratio is defined asone gram equivalent of the cyanopyridine on a cyano group basis for eachgram equivalent of sulfide to be reacted. While it is usually preferableto exceed the stoichiometric ratio of cyano groups to sulfide, sometimesit may be economical, particularly with high H 8 partial pressures (dueto high total pressure or high I-I S content), to use a lower ratio,whereby part of the sulfide loading capacity of the contacting solutionwill depend upon the solvent selected. In general, the concentration ofcyanopyridine in the contacting solution may vary from 0.1 weightpercent up to the solubility limit, preferably in the higher ranges formaximum sulfide loading of the contacting solution.

The alkali hydrosulfide used in the contacting solution is believed toact as a catalyst for the reaction of the sulfides with thecyanopyridine to form the thioamides or derivatives thereof. The alkalihydrosulfide salts such as potassium hydrosulfide, sodium hydrosulfide,lithium hydrosulfide, ammonium hydrosulfide and dimethyl ammoniumbisulfide and the like are suitable. Of the several hydrosulfides,potassium bisulfide is especially preferred because of its highcatalytic activity and in addition its ease of preparation and thermalstability. Instead of the alkali hydrosulfide itself, compounds whichare capable of forming alkali hydrosulfides in the contacting solutionunder the reaction conditions may also be used; for example, sodiumsulfide and potassium hydroxide both convert to hydrosulfides in thecontacting solution in the presence of H 8 and hence these compounds maybe used instead of the alkali hydrosulfides per se. The amount of alkalihydrosulfide present in the contacting solution preferably should bemaintained at a ratio to the amount of nitrile present of 0.01 to 0.5gram mol of hydrosulfide per gram equivalent of cyanopyridine. Generallythe upper limit on the amount of alkali hydrosulfide is determined bysolubility in the contacting solution.

Although the cyanopyridines which are liquid at operating conditions canbe used with little or no solvent, it is generally preferred to use asolvent in order to keep the ingredients and reaction products dissolvedin the contacting liquid. Such so1- vent should be an hydroxyl-freeliquid since it has been found that the presence of hydroxyl groupsinterferes with the process; it is believed that hydroxylated solventssuch as glycols react with the alkali hydrosulfide during theregeneration of the contacting solution. In other words, the solventshould be inert during the contacting and the regeneration to thereaction products as well as the selected nitrile and alkalihydrosulfide. The solvent should be able to hold in solution theselected cyanopyridine and hydrosulfides as well as the reactionproducts of the feed with the contacting solution. Also, the solventshould be thermally stable at the conditions of use.

In order to increase the reaction of the sulfides with the reagents inthe contacting solution, it is preferred that the solvent have theproperty of readily absorbing or rapidly dissolving the H 8 or othersulfides to be removed from the feed. To minimize losses throughout theprocessing cycle, it is preferred that the solvent have a relatively lowvolatility. Suitable solvents include pyrrolidones such as Z-pyrrolidone(m.p.=70F.), N-methyl 2-pyrrolidone (m.p.=l2 F.), piperidones,cyclotetramethylene sulfones such as sulfolane and dimethyl sulfonlane,lower alkylene carbonates such as propylene carbonate, benzonitrile,dialkyl ethers of polyethylene glycol such as 1,2-bis [2 methoxyethoxyl]ethane (triglyme) or bis [2-(2-methoxyethoxy) ethyl] ether (tetraglyme),and mixtures thereof. Solvents having high solvent power or goodafi'mity for H 8 are generally preferred. Of these, N-methyl2-pyrrolidone is especially preferred because of its affinity for H S,low crystallization point, low vapor pressure and dissolving power forthe reagent and reaction product.

As indicated above, the process is especially applicable to thepreferential removal of H 5 from admixtures of light hydrocarbons suchas C -C hydrocarbons, carbon dioxide and H 5. A special feature of thepresent invention is the substantial removal of H 8 from such gaseousadmixtures so that, for example, natural gas (predominantly methane)containing relatively small amounts of H per SCF) can be efficientlytreated to produce a pipeline specification gas of below 0.25 grain of H5 per 100 SCF even when the sour natural gas stream is at atmospheric ofsubatmospheric pressure. (1 grain H 8 per 100 SCF is equivalent of 15.9ppm by volume and 22.88 mg/m).

While the process has special application to treating gases having theforegoing dilute H S content, the process can be applied advantageouslyto the selective removal of H 8 and like sulfides from fluids havinghigher concentrations of these undesirable sulfides. In addition tonatural gas, other suitable feed streams include industrial gas streams(such as obtained in oil refinery operations) as well as flue gases,fuel gases and hydrogen gas streams contaminated with sulfides. Thepresent process can also be used to remove H 8 from synthais gas (i.e.,mixture of H 5 with H CO, and CO produced by partial oxidation ofsulfur-containing hydrocarbonaceous materials. A particular applicationis for the selective removal of H 8 from Claus furnace tail gases, wherethe Claus furnace is run under conditions to leave some unconverted H 8in the tail gas.

The process can be advantageously used to remove hydrogen sulfide andhydrocarbon derivatives thereof such as mercaptans. Normally thesederivatives will be lower molecular weight alkyl mercaptans and thefeeds to the process including the contaminating sulfides preferablyhave boiling ranges similar to natural gas, i.e., are gaseous atstandard conditions.

In most instances, it is desirable to have the feed substantially dry;however, the process can be applied satisfactorily to moist gases.Usually, it is preferable to dehydrate the feed and most desirably witha dehydrating agent that does not carry over into the sulfide removalsystem or that, if so carried over, does not adversely affect thecontacting solution in the sulfide removal system. A particularlyadvantageous arrangement is to use the pyrrolidones as the solvent inthe contactingsolution of the sulfide removal system and to use the samepyrrolidone in a pretreatment stage of contacting to dehydrate the feedgas. If there is any carryover of pyrrolidone from the dehydratingpretreatment, the pyrroline does not adversely affect the sulfideremoving solution and can be easily recovered.

N-methyl 2-pyrrolidone has been found to be an excellent dehydratingliquid.

The process of this invention can be carried out using contactingprocedures conventional in absorption methods wherein thesulfide-containing feed is contacted with the contacting solution eitherbatchwise or countercurrently or concurrently. While batchwisecontacting can be used, it is preferred to contact the sulfidecontaining feed in a countercurrent absorption tower with the contactingsolution in a continuous flow method. Suitable bubble cap or perforatedtrays, or packing such as raschig rings or berl saddles, or other meansof ensuring adequate and efficient contacting can be provided. Carbondioxide and light hydrocarbons which become absorbed in the solvent ofthe contacting solution are preferably first rejected in one or morestages of flashing accomplished by reducing the pressure on thesolution. A substantial portion of the aromatic and heavier hydrocarbonsabsorbed by the solvent may be rejected by simultaneously orsubsequently increasing the temperature of the contacting solution to avalue not exceeding the decomposition temperature of the reactionproducts and venting the resulting vapors.

, Thereafter, the spent contacting solution is preferably regenerated byadditional heating to raise the temperature of the solution above thedecomposition temperature of the reaction products, thereby dissociatingthe reaction products and effectively separating the sulfides from theliquid phase contacting solution.

In some instances, such as when processing gases of relatively low H 8concentrations (e.g., one-half mole percent H 8 or less), it may bepreferable to partially regenerate the spent contacting solution by asimple heat and flash cycle, thus increasing the required circulationrate of the contacting solution and thereby improving the stability ofthe contacting enriched contacting solution) depends inter alia upon themethod of contacting, rate of stripping gas, it any, temperacolumnoperation. In most instances, and particularly when processing gasescontaining relatively high concentrations of H 8 (e.g., 1 mole percent H8 or higher), it is preferable to use a conventional reboiled strippingcolumn for regenerating the spent contacting solution, thereby obtainingmore complete regeneration and increased thermal efficiency in theprocessing cycle. In other instances, and particularly when thecontacting solution contains a solvent which degrades appreciably at itsnormal boiling point, the spent contacting solution may be regeneratedby heating together with an insert gas such as nitrogen, methane, andthe like. Where the enriched contacting solution is not adverselyaffected, air can be a preferred inert gas and stripping agent. Also,contacting solu tions which contain solvents that degrade appreciably attheir normal boiling point may be regenerated by introducing a thermallystable liquid of increased volatility, e.g., benzonitrile, xylene,paraffinic hydrocarbons, and the like, into the reboiler zone andrecovering the more volatile solvent in an overhead condenser forrecirculation back to the reboiler zone. Regeneration time (i.e.,residence time of the ture, pressure, the nature and composition of thesolution, and the amount of reaction products.

The contacting step of the process is generally carried out attemperatures above that sufficient to maintain solubility of the severalcomponents of the contacting solution as well as the reaction products.The maximum temperature in the contacting zone should be that which isbelow the temperature at which the desired reaction of the sulfides withthe nitrile is reversed. Usually the temperature will be in the rangefrom about -5F. to no more than 250, depending upon the composition ofthe selected contacting solution. The pressures in the contacting stepwill be those which are practical such as from subatrnospheric to 2,000psig. Desirably the feed and contacting solution are brought together incontinuous operationsand at conditions which depend upon the sulfidecontent of the feed, the desired purity of product and the nature of thecontacting solution.

After the desired rejection of dissolved CO and hydrocarbons, such as byflashing at reduced pressure, the contacting solution is regenerated.The regeneration is carried out at higher temperatures than those usedin the contacting step for the same contacting solution. Generally thetemperature of regeneration will be the minimum required to obtaindissociation of the sulfide from the contacting solution within areasonable period of time and the pressure may be from subatrnosphericto superatmospheric. Usually the regeneration pressure is below psig,and preferably in the range of 0 to 20 psig. When the H 8 or equivalentsulfide goes to a Claus furnace, the regeneration is usually carried outat about 20 psig. The maximum temperature for regeneration is limited toprevent excessive loss or degradation of the components of thecontacting solution. In most instances the regeneration temperature willbe above 200F. For example, at atmospheric pressure, the regenerationtemperature can be in the range of 200 to 400 F depending upon thestability of the ingredients of the contacting solution.

As indicated above, H 8 released in the regeneration of the contactingsolution can be introduced into a Claus plant for conversion to sulfur,and this combination of steps is particularly desirable. Also, the H 8can be treated in accordance with the process of Keller U.S. Pat. No.3,401,101 or can be converted to sodium sulfide by contacting with anaqueous caustic solution.

Further, the contacting solution can be purified by intermittently orcontinuously withdrawing a portion of the solution and separatelytreating it to remove impurities. Altemately the solvent can bereclaimed and the solution reconstituted. For example, thecyanopyridines may be recovered by distillation.

Since the present invention involves a selective reaction of H 8 andlike sulfides with the cyanopyridine, the process can be used to removeall or any portion of the sulfide content of the feed. Thus, the processapplies to removal of H 8 in any concentration from H S-containing gasesand is particularly applicable to purification of gases having lowpartial pressures of H 8 such as below 0.1 psi of H 8. Thus the processcan remove sufficient H S, i.e., so the gas meets pipeline specification(0.25 grains H S/ 100 SCF of gas) or can remove all the H 8. Althoughthe process can be used to remove effectively H 8 from gases containinglittle or no other acidic com ponents, the process is especiallyeffective in selectively removing H 8 from gases containing appreciableamounts of CO DESCRIPTION OF THE DRAWING The invention will be morefully understood by reference to the FIGURE and the followingdescription which illustrate a preferred process flow of the presentinvention: a feed gaseous mixture such as sour natural gas containing H8 and CO is fed through line 1 into the bottom portion of contactor 2.Contacting solution such as N-methyl-Z-pyrrolidone containing 78.46 wt.percent of mixed ortho and meta cyanopyridine and 0.92 wt. percent ofpotassium hydrosulfide, is introduced into the upper portion ofcontactor 2 through line 3. Contactor 2 can be any suitable contactingcolumn containing appropriate packing or trays to assure intimatecountercurrent contact of the rising gaseous feed with the downwardlyflowing contacting solution. Contactor 2 is maintained under suchconditions of temperature and relative flow rates so that H 8 isselectively reacted with substituted aromatic cyanopyridine within thestoichiometric limit, CO being absorbed by the solvent to the extentdetermined by the conditions and amounts present.

Gas of substantially reduced H 8 content is removed from the contactorand sent to thesales gas pipeline as purified gas via line 5 throughvalve 4, valve 6 being in a closed position.

The sulfide-enriched contacting solution is withdrawn from contactor 2through line 7. If the withdrawn solution contains substantial amountsof dissolved CO and/or hydrocarbons, as may occur at the highercontacting pressures, the solution can be passed through an expansionvalve 8 to flash zone 9 for controlled pressure reduction to vaporizesuch dissolved CO and/or hydrocarbons which are removed overhead vialine 10, either for recycle to contactor 2 through line 11 andcompressor 12, or other disposal via line 13.

The liquid withdrawn via line 14 from flash zone 9 (or, in the event theflash zone is omitted, the sulfide-enriched solution withdrawn fromcontactor 2) is passed through heat exchanger 15 into a second flashzone 16 where additional hydrocarbons and residual CO can be removed vialine 17 for suitable disposal. The liquid withdrawn via line 18 ispassed to an intermediate point in a conventional stripping column 19which is equipped with a reboiler 20 and which is operated underconditions to substantially regenerate the contacting solution. The HS-rich gas stream is removed overhead by means of line 21, passedthrough condenser 22 where vaporized solvent and hydrocarbons arecondensed, then routed to reflux drum 23. The liquid phase iscontinuously removed from drum 23 by means of line 24 and returned viapump 25 to the top of column 19 to serve as reflux. Hydrocarbons whichare not miscible with the condensed solvent can be withdrawn eithercontinuously or intermittently via line 26 for suitable disposal.Concentrated ms is removed from drum 23 via overhead line 27 forappropriate disposal such as to a Claus furnace or acid plant. Theregenerated contacting solution is withdrawn from the bottom of column19 through line 3 and returned via pump 28 to column 2, passing in turnthrough heat exchanger 15 and cooler 29 which reduce the temperature ofthe solution to the desired contacting temperature. Make-up solution maybe added via line 30.

When it is desired to reduce the CO content of the sulfidefree gasstream 5 below that obtainable with the stoichiometric circulation rateof contacting solution before mentioned (e.g., particularly when the COcontent and pressure of sour feed gas stream 1 are high), thesulfide-free gas is preferably passed to a second contactor 52 via valve6 and line 51, valve 4 being closed. To avoid contamination of solvents,and to assist in the possible recovery of solvent from the gas streamleaving the first contactor, it is most desirable to employ the sameliquid for a solvent in the contacting solution used in column 2 as isused for the absorbent in column 52. A suitable liquid for such purposeis, for example, N-methyl-2-pyrrolidone. In the second contactor 52,suflicient solvent is circulated to absorb the additional amount of COnecessary to reduce the C content of the H S-free gas leaving contactor52 via line 54 to the desired value. The solvent containing CO withdrawnfrom contactor 52 is passed via line 55 through pressure reducer 56 toflash drum 57 (or other suitable solvent-CO separator) and CO iswithdrawn for disposal through line 58. Regenerated solvent is recycledby means of line 53 and pump 59 back to the top of contactor 52.

When the H 8 content and pressure of sour feed stream are low (e.g.,less than 0.5 mol percent H S and 100 psi), regeneration of thecontacting solution may be accomplished preferably by a simple heat andflash cycle. In such cycle, all

or a portion of the sulfide-rich contacting solution from the bottom ofcontactor is flashed to essentially atmospheric pressure through heatexchange to a heater wherein the temperature of the solution is raisedto a value above that at which the reaction product decomposes, thenceinto an appropriate flash drum where H 8 is removed via an overheadstream for suitable disposal and the partially regenerated contactingsolution is removed via a bottom stream and pumped back to the top ofcontactor through suitable heat exchange and cooling. The circulationrate of contacting solution through the regeneration cycle depends interalia upon the regeneration temperature and residence time as well as thepressure and H 8 content of the sour feed stream. That portion of thesulfide-rich contacting solution which is not subjected to regenerationis recycled back to the top of the contactor where it is mixed with thepartially regenerated solution prior to entering the column.

In the foregoing description in reference to the figure, variousauxiliary equipment and processing alternatives have been eliminated forthe sake of simplicity. For example, in place of expansion valves 8 and56 turbo expanders can be used with the power offtakes arranged to drivesolution pumps 28 and 59, to recompress flash gas streams 13 and 58, orlike means of expansion energy recovery. Also, the sulfide-rich and CO,-rich contacting solution pressures may be reduced in two or more stages,thereby enhancing power recovery or optimizing heat exchange. Further,appropriate heat exchange may be desirable at each expansion stage toavoid low solution temperatures resulting from isenthalpic or isentropicexpansion. Also, regeneration of the sulfide-rich contacting solutioncan be assisted by introducing an inert gas or vapor into the lowerportion of the regeneration column. Further, a slip stream of theregenerated contacting solution can be withdrawn continuously orintermittently for removal of accumulated minor impurities and purifiedas indicated above.

In some instances (e.g., at low processing pressures or when arelatively volatile solvent is used in the contacting solution),supplemental means may be desirable for recovering vaporized solventfrom the sweetened gas stream. One suitable means consists of intimatelycontacting the sweetened gas stream with a liquid which is miscible withthe solvent of the contacting solution, separating the solvent-richliquid from the sweet gas, and subsequently distilling the liquidmixture to recover the solvent. When using the preferred solvent of thepresent invention, N-methyl-2-pyrrolidone, the wash liquid can be waterin which case the solvent is recovered as a kettle product in thedistillation step, or the wash liquid may be sulfolane in which case thesolvent is recovered as a distillate in the distillation step. Ingeneral, a nonaqueous solvent of low volatility is preferred as the washliquid so that the dehydrated gas will not be resaturated with water.Another method for accomplishing the desired solvent recovery consistsof chilling the sweetened gas stream and recovering a substantialportion of the vaporized solvent as a condensate product. Either methodcan also be applied to recover solvent from the concentrated H 8 streamproduced in the regeneration of the sulfide-rich contacting solution.

The following examples further illustrate the process of the presentinvention EXAMPLE 1 A solution of 78.46 wt. percent of mixed 2- and 3-cyanopyridines (approximately a 50-50 mixture) in N-methyl 2-pyrrolidonetogether with 0.017 gram mol of potassium hydrosulfide per gramequivalent of cyanopyridine was prepared. This solution had acrystallization point of about 0 F. Then the solution was tested forsweetening activity with sour natural gas of varying H 8 content asfollows: ml. of the contacting solution under test was placed in agas-absorption bottle having a fritted porcelain disk near the bottomand above the gas inlet to insure uniform gas distribution into theliquid. Ceramic berl saddles of 1 cm. were also placed in the bottleabove the fritted disk to promote gas-liquid contact. The sour naturalgas stream was bubbled through the solution at a rate of 250-280 cc/min.A bypass line is provided to enable analysis of the feed gas streamwithout passing through the contacting solution in the gas absorptionbottle. With the aid of suitable valving, the feed gas or the treatedgas stream was analyzed in a continuous manner by a calibrated automaticrecording H S analyzer. Thus, the feed gas passed through the bypassline was first analyzed; then, after changing the valving to close offthe bypass line and to direct the gas through the absorption bottle, andafter allowing about 2 minutes for displacement of inert gas from thebottle, the treated gas was analyzed. By varying the l-l S content ofthe inlet gas and measuring the 11 s content of the treated gas, dynamicabsorption equilibrium was determined for each l-l S content. Thepressure on the gas was slightly over atmospheric and the temperaturewas ambient, i.e., about 72-74 F. The following results were obtained:

TABLE I Grains H,S/ 100 SCF Gas Feed H,S Conc. Ratio Inlet Gas OutletGas Inlet/Outlet These results illustrate the effectiveness of themixture of ortho and meta cyanopyridines in the contacting solution forremoval of sulfide from the feed. Also to be noted is the highsolubility of the cyanopyridines in N-methyl 2-pyrrolidone.

EXAMPLE 2 Another solution was made up of 50.1 percent mixed 3- and4-cyanopyridines (about a 50-50 mixture) in N-methyl 2-pyrrolidone with0.017 gram mol of potassium hydroxide per gram equivalent ofcyanopyridine. The solution had a crystallization point of about F. andwas tested for sweetening activity in the same manner as in Example 1.The results obtained were as follows:

TABLE II Grains H,s/100 SCF Gas Feed H,S cone. Ratio lnlet Gas OutletGas Inlet/Outlet The above results illustrate the effectiveness of themixture of meta and para cyanopyridines in the contacting solutions forremoving H 8 from sour gas.

EXAMPLE 3 A solution of 40.5 percent 3-cyanopyridine in N-methyl 2-pyrrolidone along with 0.017 gram mol of potassium hydrosulfide per gramequivalent of cyanopyridine was prepared. The solution had acrystallization point of about 0 F. and was tested for sweeteningactivity in the manner described in Example l. The results obtained wereas follows:

TABLE Ill Grains H,S/l00 SCF Gas Feed ",8 cone. Ratio Inlet Gas OutletGas lnletJOutlet The above results illustrate the removal of H28 fromsour natural gas in one stage of treatment by means of 3- cyanopyridinein the contacting solution. By using additional statttg of contactingmore Hg can be removed.

er tests earned out wr solutions of the cyano-pyndtnes without thehydrosulfide showed that such solutions were inactive.

Numerous other examples can be given to illustrate the present inventionand its many applications. For example, a particular applicationinvolves treating an fi s-containing gas in a pipeline by concurrentlycontacting said gas with one of the contacting solutions describedhereinabove and providing for accumulation of the r-ns enriched solutionsuch as in sumps, knockout drums, or the like so that the treated gas ofreduced H S content can continue on and the enriched solution removedfor regeneration and recycle as desired.

We claim:

l. A process for removing hydrogen sulfide from a gas mixture containingsaid hydrogen sulfide, which process comprises contacting said gasmixture with a liquid contacting solution containing a cyanopyridine andan alkali hydrosulfide selected from the group consisting of potassiumhydrosulfide, sodium hydrosulfide, lithium hydrosulfide, ammoniumhydrosulfide, and dimethyl ammonium hydrosulfide, or compounds capableof producing one of said hydrosulfides, and separating the hydrogensulfide-enriched contacting solution from a resulting gas stream ofreduced hydrogen sulfide content.

2. A process for removing hydrogen sulfide from a gas mixture containingsaid hydrogen sulfide, which process comprises contacting said gasmixture with a cyanopyridine and an alkali hydrosulfide selected fromthe group consisting of potassium hydrosulfide, sodium hydrosulfide,lithium hydrosulfide, ammonium hydrosulfide, and dimethyl ammoniumhydrosulfide, or compounds capable of producing one of saidhydrosulfides in a substantially hydroxyl-free solvent, saidcyanopyridine, hydrosulfide and solvent constituting contacting solutionand separating the hydrogen sulfide-enriched contacting solution from aresulting gas mixture of reduced sulfide content.

3. The process of claim 2 wherein said contacting solution isregenerated by heating and removing the dissociated hydrogen sulfide.

4. The process of claim 2 wherein said solvent is a pyrrolidone.

5. The process of claim,2 wherein said solvent is N-methyl-2-pyrrolidone and said contacting is carried out at a temperature belowabout 250 F.

6. The process of claim 2 wherein said cyanopyridine is a mixture ofortho and meta cyanopyridines.

7. The process of claim 2 wherein solvent initially contains a compoundcapable of producing an alkali hydrosulfide under the reactionconditions.

8. The process of claim 2 wherein said alkali hydrosulfide is potassiumhydrosulfide.

2. A process for removing hydrogen sulfide from a gas mixture containingsaid hydrogen sulfide, which process comprises contacting said gasmixture with a cyanopyridine and an alkali hydrosulfide selected fromthe group consisting of potassium hydrosulfide, sodium hydrosulfide,lithium hydrosulfide, ammonium hydrosulfide, and dimethyl ammoniumhydrosulfide, or compounds capable of producing one of saidhydrosulfides in a substantially hydroxyl-free solvent, saidcyanopyridine, hydrosulfide and solvent constituting contacting solutionand separating the hydrogen sulfide-enriched contacting solution from aresulting gas mixture of reduced sulfide content.
 3. The process ofclaim 2 wherein said contacting solution is regenerated by heating andremoving the dissociated hydrogen sulfide.
 4. The process of claim 2wherein said solvent is a pyrrolidone.
 5. The process of claim 2 whereinsaid solvent is N-methyl-2-pyrrolidone and said contacting is carriedout at a temperature below about 250* F.
 6. The process of claim 2wherein said cyanopyridine is a mixture of ortho and metacyanopyridines.
 7. The process of claim 2 wherein solvent initiallycontains a compound capable of producing an alkali hydrosulfide underthe reaction conditions.
 8. The process of claim 2 wherein said alkalihydrosulfide is potassium hydrosulfide.